It seems much more difficult now. In the legacy sims one had easy access to the config and wav files. While the legacy sound system is still (mostly) supported, there's a lot more involved. The prime path is now Wwise audio, which has a more-or-less encrypted sound bank. I've seen a few freeware cases where developers have done a hybrid -- added and/or replaced some sounds using legacy calls in conjunction with a default Wwise sound bank, so that may a way to go. The SDK documents both systems and has examples as well.
Despite the extra effort I think Asobo made the right choice in pushing forward with Wwise, as it is leaps and bounds beyond what the legacy system could do. But it makes it much less approachable for the average user to change sounds.
- dcc